The increased use of electromagnetic waves for communication has spawned a crowding of the electromagnetic spectrum, and has led to a pressing need for the design of reliable high frequency electronic components from which equipment for utilizing the higher frequencies of the RF spectrum may be constructed. A common feature of many electronic communication devices is the need for components which exhibit a variable impedance. Devices as simple as potentiometers used in volume controls, and multiplate ganged variable capacitors, have long been used for effecting mechanically driven changes in impedance. The multiplate ganged capacitor was for many years the standard variable reactive impedance used in tuning radios.
More recent developments have led to a large market for devices exhibiting a reactive impedance which may be electronically changed. While these devices have a multiplicity of uses, one of their most common uses is that of a voltage variable reactance as a frequency determining element in an oscillator circuit. The result is, of course, a variation in the output frequency of the oscillator.
For many years such an arrangement has been a staple component of television receivers. The most widely used embodiment of electronically variable reactive impedances is a device known as the varactor diode. These have been widely used in tuning circuits and automatic frequency control circuits since the 1960's. More recently, as phase lock loop frequency synthesis has become more popular, varactor diodes are used as components of the voltage controlled oscillators in phase lock loop tuning circuits.
While varactors have been widely used, and continue to enjoy a great popularity, there are a number of drawbacks to the use of varactors. Some of the constraints of semiconductor physics irrevocably lead to some undesirable properties in varactors. At least one result of these constraints is that varactors are characterized by relatively low quality factors (Q). As the design of electronic circuitry requiring tuned reactive components, and a particularly tuned capacitor, extends into higher frequency ranges, the problems associated with the relatively low Q of varactors become exacerbated.
While varactors are extremely useful devices and will maintain an established place in the electronic arts in the near term, they exhibit well known deficiencies as a result of the physics of their fabrication. As the need for higher and higher frequency communication circuits increases, it will be appreciated that more and more applications for electronically variable reactances present themselves for which varactor diodes and other prior art devices are unsuited.
Thus, there is a strong need in the current state of the art for a device which can inexpensively replace varactor diodes, in the common combination of fixed or mechanically changeable capacitor in parallel with a varactor, and which will exhibit relatively high Qs at frequencies outside the practical operating range of inexpensive varactors.
In the past, a number of variable capacitance arrangements have been proposed using piezoelectric elements to vary the spacing between capacitor plates. For example, U.S. Pat. Nos. 2,368,643 and 3,646,413 show variable capacitors which may be varied by a voltage applied to produce an electric field through a piezoelectric element. The piezoelectric element is mounted on a cantilever to enhance the mechanical movement and thus enhance the change in capacitance. These arrangements appear to be susceptible to "creep" and a significant degree of thermal instability. The disclosure of U.S. Pat. No. 3,646,413 shows complex geometries, and an electronic feedback circuit to compensate for the mechanical instabilities of the cantilevered piezoelectric arrangement. This tends to increase the cost of such a device.
U.S. Pat. No. 3,949,246 shows a voltage variable capacitor, each plate of which consists of a metalized deposit on a piezoelectric bimorph. A control voltage to vary the capacitance is provided to cause the bimorphs to flex, thus bringing the plates closer together, increasing the capacitance of the arrangement. The structure of the device disclosed in U.S. Pat. No. 3,949,246 inherently provides a large contact area between the two bimorph elements carrying the capacitor plates. As is known to those skilled in the art, the interface between the bimorph elements acts as a shunt across the capacitor plates. It appears to the inventor of the present invention that this device would also suffer from unacceptably low quality factors as the frequency applied to the capacitor terminals exceeds ten megahertz.
In practical applications, prior art devices, both varactors and prior art piezoelectric capacitors usually require either a fixed or mechanically adjustable capacitor to be placed in parallel with the variable element to set the midpoint of the desired tuning range. This increases the number of parts necessary to construct a circuit making practical use of such devices. Additionally, varactor diodes, being two terminal devices, of necessity have both the control voltage and the alternating voltage of interest applied between the same terminals. Since in most applications of varactors, it is desired to vary the control voltage without causing additional distortion to the alternating signal of interest, circuits using varactors normally require that the magnitude of the alternating voltage be significantly smaller than that of the control voltage lest the capacitance of the device begin changing at a frequency corresponding to the alternating frequency of interest.
Additionally, any significant amplitude modulation of the signal controlled by the varactor will lead to changes in capacitance which can thus impose an unwanted modulation of the frequency upon the signal.